1. Field of the Invention
The present invention relates to a noise suppression method, a noise suppression method program, a recording medium recording a noise suppression method program, and a noise suppression apparatus. More particularly, the present invention can be applied to a time-cyclic noise filter eliminating noise of a video signal. The present invention makes it possible to suppress noise more sufficiently and at a higher speed than in the past by counting the number of fields or the number of frames after a sudden change in the signal level of an input video signal, and dynamically controlling the feedback ratio to increase gradually in accordance with the count value.
2. Description of the Related Art
To date, as shown in FIG. 13, a time-cyclic noise filter has been used to suppress noise of a video signal using a field difference of a frame difference. That is to say, in this noise filter 1, a delay-signal generation section 2 delays an output video signal S2 for a period of one field or one frame, and outputs a reference video signal S3 for extracting noise components. In this regard, the reference video signal S3 may be generated by performing motion compensation on the output video signal S2 here. A subtraction circuit 3 subtracts the reference video signal S3 from the input video signal S1 to generate a difference signal S4. A compensation-signal generation section 4 multiples the difference signal S4 by a feedback ratio k to generate a noise compensation signal S5. In this regard, at this time, the noise compensation signal S5 is sometimes generated such that, for example, the smaller the amplitude of a change is, the more likely that the change is noise, and the feedback ratio is set to a high value, on the contrary, the larger the amplitude of a change is, the more likely that the change is not noise, and the feedback ratio is set to a low value. Also, the difference signal S4 is sometimes bank-divided and processed. A subtraction circuit 5 subtracts the noise compensation signal S5 from the input video signal S1 to generate the output video signal S2.
On such a time-cyclic noise filter, a scheme for measuring a noise level and automatically setting the signal level of the noise compensation signal S5 on the basis of the noise-level measuring result has been proposed in Japanese Unexamined Patent Application Publication No. 2001-136416, etc.
The noise reduction processing in such a time-cyclic noise filter can be expressed by the following recurrence relation. In this regard, here, It is the signal level of the input video signal S1, and Ot is the signal level of the output video signal S2. Also, the subscript t of each symbol is time. When the processing unit of the input video signal S1 is a frame, or a field, the subscript t is the number of the frame, or the field from the start, respectively. Accordingly, in the configuration in which the output video signal S2 is simply delayed and fed back, Ot-1 is the signal level of the reference video signal S3, and (It−Ot-1) is the signal level of the difference signal S4. In this regard, here, it is assumed that the average signal level of the input video signal S1 is 0 level, and this input video signal S1 includes only a noise component having variance σ02.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                                                                O                t                            =                                                I                  t                                -                                  k                  ·                                      (                                                                  I                        t                                            -                                              O                                                  t                          -                          1                                                                                      )                                                                                                                          =                                                                    (                                          1                      -                      k                                        )                                    ·                                      I                    t                                                  +                                  k                  ·                                      O                                          t                      -                      1                                                                                                                              (        1        )            
The signal level of noise average 0 level. There is no correlation between successive fields or frames. Thus, the variance σ02 of the reference video signal S3 at time t can be expressed by the following expression on the basis of Expression (1).
[Expression 2]σ02=(1−k)2·σ02+kσt-12  (2)
By solving the recurrence relation of Expression (2), the variance of the output video signal S2 at time t can be expressed by the following general expression.
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      σ          t          2                =                                            (                                                2                  ·                                      k                                                                  2                        ⁢                        t                                            +                      1                                                                      +                1                -                k                            )                                      (                              1                +                k                            )                                ·                      σ            0            2                                              (        3        )            
Here, since the feedback ratio k is less than 1(k<1), when time t is infinity, the variance2 of the output video signal S2 can be expressed by the following expression.
                    [                  Expression          ⁢                                          ⁢          4                ]                                                                      σ          ∞          2                =                                            (                              1                -                k                            )                                      (                              1                +                k                            )                                ·                      σ            0            2                                              (        4        )            
Here, σt2/σ02 represents the noise reduction rate at time t. Accordingly, the noise reduction rate at infinite time by the feedback ratio k is shown by FIG. 14. Thus, from Expression (4) in FIG. 14, it is understood that in a known time-cyclic noise filter, the noise reduction effect becomes higher as the feedback ratio k is set higher.
From the above relational expression, the relationship between the variance σt2 of the output video signal S2 and time t is shown in FIG. 15. Accordingly, it is understood that, in the known time-cyclic noise filter, if the feedback ratio k is set high, it takes time to converge.
Thus, in the known time-cyclic noise filter, there has been a problem in that if the setting is determined so as to increase the noise reduction effect, it is difficult to ensure the noise reduction effect of a portion in fast motion. That is to say, for example as shown in FIG. 16, if moving objects 7A and 7B are moving in front of a still background at a high speed, when viewing a part of the background, indicated by the arrow A, over which the moving objects 7A and 7B are crossing, the background appears and the moving objects 7A and 7B appears in this part at points t1, t2, and t3 in time as shown in FIGS. 17 and 18. In this case, if the feedback ratio k is set to a low value, noise can be reduced sufficiently at a high speed in response to the moving speed of the moving objects 7A and 7B as shown in FIG. 17, whereas the noise reduction effect becomes little. Also, in this case, in the part of the background over which the moving objects 7A and 7B are not crossing, it becomes difficult to sufficiently ensure the noise reduction effect because the feedback ratio k is low.
On the contrary, if the feedback ratio k is set to a high value, in the part of the background over which the moving objects 7A and 7B are not crossing, it is possible to ensure the noise reduction effect sufficiently. However, in the part of the background over which the moving objects 7A and 7B are crossing, it becomes difficult to suppress noise sufficiently at a high speed in response to the moving speed of the moving objects 7A and 7B as shown in FIG. 18.